I. ˈən-dər-ˌlīn, ˌən-dər-ˈ transitive verb
Date: 1721
1. : to mark (as a word) with a line underneath
2. : to put emphasis on : stress
3. : to show clearly or emphatically
II. ˈən-dər-ˌlīn noun
Date: 1886
1. : the outline of a quadruped's underbody ; also : the ventral surface of a quadruped's body
2. : a horizontal line placed underneath something